In many push-to-talk (PTT) systems, an originating mobile station typically sends a Session Initiation Protocol (SIP) INVITE message over a communication channel to initiate the establishment of voice communications with a target mobile station. This SIP INVITE message causes a dispatch server in the network to page the target mobile station. The target mobile station then responds to the page and obtains a dedicated data traffic channel to communicate with the originating mobile station.
In these previous systems, after the data traffic channel is established, the target mobile station typically waits until it receives the SIP INVITE message from the server before returning a 200 OK message to the dispatch server. Then, a talk permit tone (TPT) is sent to the originating mobile station from the dispatch server to initiate the actual voice transmission over a voice communication channel. Once the TPT has been received at the originating mobile station the speaker begins speaking, the voice communication channel can be established between the originating mobile station and the target mobile station and voice communications can occur.
The approaches used in previous systems result in significant delays in establishing voice communications between mobile stations in a network. For instance, delays occur because of the transmission of the SIP INVITE message to the target mobile station. Processing time is also required at the target mobile station to process the SIP INVITE message. An Automatic Repeat Request (ARQ) such as that incurred by Radio Link Protocol (RLP), Radio Lick Control (RLC), or Hybrid ARQ (HARQ) round-trip-delay also exists. Additional delays occur since the target is required to generate the 200 OK message and to transmit this message to the originating mobile station. If SIP compression is used, the delays can be still longer as time is required to perform compression and/or decompression on the various messages. All of these factors lead to a degradation of system performance and a potentially unpleasant user experience due to the delays.
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